1. Field of the Invention
The invention relates generally to signal processing techniques for fiber optic interferometric sensor systems. More particularly, the invention relates to a method and system of using odd harmonics for Phase Generated Carrier (PGC) homodyne technique that utilizes an eight-point or sixteen-point Fast Fourier Transform (FFT) demodulation algorithm.
2. Description of the Related Art
The present invention is related to application Ser. No. 10/600,099, entitled “CALCULATION OF SENSOR ARRAY INDUCED PHASE ANGLE INDEPENDENT FROM DEMODULATION PHASE OFFSET OF PHASE GENERATED CARRIER,” application Ser. No. 10/615,729, entitled “FILTERED CALCULATION OF SENSOR ARRAY INDUCED PHASE ANGLE INDEPENDENT FROM DEMODULATION PHASE OFFSET OF PHASE GENERATED CARRIER,” and U.S. patent, U.S. Pat. No. 6,944,231, entitled “DEMODULATION OF MULTIPLE-CARRIER PHASE-MODULATED SIGNALS,” assigned to the assignee of the present invention. These two applications and patent are incorporated herein by reference.
Acoustic sensor systems for underwater applications are well known in the art. In seismic or oil exploration, these sensor systems are typically employed in static arrays of multiple acoustic transducers that are placed on or beneath the sea floor. Each sensor array reacts to acoustic pressure waves, initiated from a surface ship, by modulating an input signal and the collected data is then processed and analyzed to determine optimum drilling locations or to monitor undersea geological structures. In military surveillance, these sensor systems are usually mounted to a submarine hull or towed behind the submarine. The sensor system provides underwater listening capabilities and relative position information.
One sensor system that employs modulation techniques is fiber optic sensors. The fiber optic sensor arrays have sensing elements, for example, fiber optic interferometers. The signals from these sensing elements are often multiplexed, by way of example, using time division multiplexing (TDM), frequency division multiplexing (FDM) and/or wavelength division multiplexing (WDM). In TDM, signals from various sensor arrays are carried on a single transmission path by interleaving portions of each signal in time. FDM simultaneously modulates different carrier frequencies on the same medium by allocating to each signal a different frequency band, while WDM involves multiplexing multiple wavelengths on a single fiber.
Typically, in TDM systems, a modulated optical signal is input to the sensor array and various demodulation techniques have been proposed and are used for correlating the signals from the array of sensors that produce the signals. Techniques providing sensing information encoded on carrier signals include phase-generated carrier (PGC) homodyne, PGC synthetic heterodyne and differential delay heterodyning.
Common to all demodulation methods for fiber optic interferometric arrays, is the acquisition of an in-phase term proportional to the cosine of the interferometer phase shift and a quadrature term proportional to the sine of the interferometer phase shift. The sine of the sensor phase shift is known as the quadrature term Q; and the cosine of the sensor phase shift is referred to as the in-phase term I. The angle of the phase shift is determined by calculating the ratio of Q/I, which is the tangent of the sensor phase shift. The amplitudes of the sine and cosine terms must be set equal by a normalization procedure to ensure the successful implementation of an arc tangent routine to find the sensor phase shift.
Existing demodulation algorithms, such as Optiphase Inc.'s six step algorithm, are dependent on a predetermined phase offset associated with the sampling of the phase generated carrier. Also, existing demodulation algorithms use a slow PGC at tens of kilohertz that requires the interrogation of multiple pulses (six for the Optiphase method) at different times to acquire one acoustic data point for a given interferometric sensor (hydrophone) return.
Assignee recently developed an eight point (samples at 45° intervals) and a sixteen point (samples at 22.5° intervals) Fast Fourier Transform (FFT) demodulation algorithms that decouples the algorithm performance from the phase generated carrier demodulation phase offset. Typically, the frequency of the PGC in this case is 5-10 MHz. The eight point FFT demodulation algorithm operates with a sampling rate that is eight times that of the PGC frequency. Such high sampling rate often places great demands on the sampling circuitry. Because an inverse relationship exists between the PGC frequency and the drive voltage, such as for a lithium niobate fiber pigtailed phase modulator, the demodulation algorithm operating at low frequency requires excessive drive voltage. For example, at 10 MHz, a peak to peak voltage of 10 volts suffices, whereas at 2.5 MHz, an excessive peak to peak drive voltage of 40 volts is required.
Since the trend in the industry is to lower power consumption for certain applications of fiber optic sensor arrays, the resulting sample rate of the eight point FFT demodulation algorithm can be reduced to 20 Mega samples per second or less to make use of low power analog to digital converters at the receiver. This, in turn, lowers the PGC frequency to 2.5 MHz or less, which ironically requires excessive drive voltage. Measures such as resonant tank circuits or transformers are less than satisfactory solutions to the problem. Consequently, there remains much scope in the art for interrogating desirable PGC frequencies at low drive voltages. Therefore, there is a need in the art for a signal processing system providing fast interrogation of sensor pulses with low drive voltages, preferably not to exceed 10-15 volts peak to peak.